High frequency oscillating circuit



Nov. 2, 1937. N. M. RUST HIGH FREQUENCY OSCILLATING CIRCUIT Filed Oct.8, 1935 2 Sheets-Sheet 1 Jlll III INVENTOR NOEL MEYER R057 BY 0 a ,jx'p/wwx/ ATTORNEY HIGH FREQUENCY OSCILLATING CIRGUI-T Filed Oct. s, 1955 2Sheets- Sheet 2 lNVENTOR ATTORNEY l atented Nov. 2, 1937 UNITED STATES]HIGH FREQUENCY osoILL G CIRCUIT Noel Meyer Rust, Chelmsford, Essex,England, assignor to Radio Corporation of America, a corporation ofDelaware Application October 8, 1935, Serial No l4,045

' In Great Britain September 14, 1934 1 Claim." (01. 250-20) Thisinvention relates to electrical high frequency oscillating circuitarrangements and has for its object to provide improved means wherebythe tuning or natural frequency of a. high 5 frequency electricaloscillating circuit may be automatically varied in dependence upondepartures from predetermined desired results in such manner as tocorrect for such departures,

The principal applications of they invention are 10 (1) to so-calledhomodyne receivers, i. e. re-

ceivers wherein a received modulated carrier wave is mixed with alocally generated carrier.

wave of like frequency for reception purposes, and (2) to what aresometimes termed self-tuning? l5 receivers, i. e. receivers wherein atuning instrumentality is caused automatically to be varied in suchmanner as to maintain the receiver accurately tuned to a desired signaleven if said signal drifts i. e. varies slightly, in. frequency; 20 Theinvention is. based upon the fact that .by' virtue of so-called Millereffect the space between the electrodes of an electron. discharge devicemay be arranged to present a substantially pure reactance of a magnitudewhich can be varied by 25 varying the operating constants of thedevice,,

and in carrying out this invention a reactance so presented is utilizedas a variable reactance in' a high frequency tuned circuit to beautomatically controlled; the operating constants of said de- 30 vicebeing varied in dependence upon departures from results desired to beobtained, such varia-' tions being such as to produce variations intuning in a sense to compensate for the departures. The invention isillustrated in the accompany:

35 ing drawings which show diagrammatically dif-.

ferent embodiments thereof. Fig. 1 shows a homodyne circuit embodyingthe invention; Fig.

2 shows a portion of a superheterodyne receiver embodying the inventioninan oscillator fre-. 4.0 quency control circuit; Fig. 3 illustrates amodification of Fig. 2. Referring to Figure 1 which showsone ar-'rangement in accordance with the invention as applied to aso-calledhomodyne receiver the local 45 oscillator of the said receiverin the particular embodiment illustrated includes certain of ftheelectrodes of a so-called heptode valve l, i. 'e; a valve having acathode 2, an anode 3 and five grids 4, 5, 6, I, 8, in successionbetween cathode 50 and anode. In the description which follows valvegrids will, for the sake of brevity,be dew scribed in reference to theirorder as-counted, from the cathode; for example, the first'grid'is thegrid 4 nearest the cathode, the second grid 55 is the next'grid 5 and soon. The first grid 4 is connected to thecathode 2 through a leakresistance 9 and is'also connected through a coupling condenser l and aparallel tuned circuit consisting of an inductance l I shunted by avariable condenser l2 to the negative terminal l3 of the anode potentialsource (not shown). The second grid 5 is connected through a coil 14which is adjustably coupled to the coil II to a source of positivepotential (not shown) connectedjat [5, the cathode 2 and the first andsecond grids 4 and 5 accordingly constituting the electrode system of alocal oscillator which is utilized as the local oscillator of a homodynereceiver, A third coil I6 coupled to the coil i I is utilized to takeoff local oscillatory energy 15' to the homodyne detector (not shown).The third and fifth grids 6, 8, are connected together and to a suitablesource of positive potential connected at I1. The fourth grid 1 iscoupled through a coupling condenser l8 to a point (not shown) of thereceiver from which received high frequency signal voltage (for lockingpurposes) is obtainable, the said grid 7 being also connected through aresistance l9 to a variable tapping point upona potentiometer resistance2| one end ofwhich is connected to the negative terminal 22 of asuitable bias source and the other endof which is connected to thecathode 2. The said cathode is connected through a milliammeter23in-series with a choke 24 and a second potentiometer resistance 25 tothe terminal l3. The potentiometer resistance 25 is shunted by acondenser 26 and a second condenser 21 is shunted across the seriesconnected choke 24 and resistance 25'the said choke and two condensersconstituting a low pass filter across which is effectively shunted theresistance 25. An adjustable tapping point 28 upon the resistance 25 isconnected through a further resistance 29 to the control grid'30 of avalve 3| which is utilized as 40 a variable reactance device, the grid30 being also connected directly to the junction point between theparallel tuned circuit H, l2 and the condenser lllii In the embodimentillustrated a pentode valve is utilized as the variable reactance deviceand the, control grid 30 is connected to the anode 33 thereof through avariable condenser 32 which servesto increase the anode-grid capacity.of the valve and which-can be varied to adjust the value of theapparent input reactance of the valve 3|.

The 'second grid 34 of the valve 3! receives positive bias from anyconvenient source (as shown it is connected to terminal I!) and thecathode"35 of the valve 3| is connected as in the usual way to the thirdgrid 36 thereof and also to an adjustable tapping point 37 on aresistance 3i? shunted between the positive and negative terminals H andI3 respectively. The anode 33 of the valve 3! is connected throughfrequency expected to be present and desired to.

be received;

Now it will be appreciated that with this ar-- rangement the gridcathode-circuit 30 ?35 of thevalve 3! is effectively in shunt across theparallel;

'. tuned circuit ll, 52, in the circuit of the first grid 4 of theheptode, and accordingly, the capacitative reactance presented by thecontrol grid circuit of the valve 3! will form a smallcondenser in shuntacross the normal condenser of the said tuned circuit.

The low pass filter will pass on to the second potentiometer 25, 23(from which potential for the control grid of the valve 3! is" obtained)either slow beats due to the interaction, between the local oscillationsand locking signal oscillations or--if the' 'local oscillator isoperating accurately at signal frequency-a resultant direct currentdepending for its magnitude upon the relative phase between the localoscillations and the locking signal oscillations. This resultant-slowbeats or direct currentwill be indicated by the milliammeter 23 andbecause of the fact'that a desired proportion of the said resultant isfed to the control grid 39 of valve 31', it is possible so to adjust'theapparatus that the tuning of the local oscillatorfwill be automaticallyvaried soas to maintain it automatically substantially at the samefrequency as the received locking signal frequency and in apredetermined phase relationship therewith. In other words, thearrangement effectively interlocks the local oscillation frequency withreceived signal oscillation frequency. 'It is found in practice possibleso to make the adjustments asto secure any relative phase relationshiprequired between local oscillations and received signal oscillations,such adjustments being made either by varying the position of thetapping point 28v or by manually adjusting the frequency of the paralleltuned circult ll, l2. Where the receivedsignals are weak the adjustmentsbecome more delicate and it is found better to move the slider 28 insuch direction as to give maximum controlling action upon the pentodeand then to adjust the tuning of the parallel tuned circuit ll, 12

An important advantage of the above described arrangement isto be foundin the fact that not only is an easily adjustable and satisfactorylooking action obtained between received, signal oscillations and localoscillations, but the local oscillations are only very weakly if at all.modulated by modulation energy from the received. signals, and this, ofcourse, is of considerable importance in homodyne receivers. The platecircuit of the he-ptode may be used for anypurpose.for example it may beconnected to an arrangement for indicating beats so as to facili tateadjustment of the'circuit. In Figure lsuch an arrangement isillustrated, and comprises a valve ll coupled as shown to the platecircuit of the heptode and giving anindicating output which may be takenfrom terminals 42.'

Figure 2rshows another embodiment .of the invention as applied to asuperheterodyne receiver wherein self-tuning is resorted to. Here apoint (not shown) in the intermediate frequency amplifier of thereceiver and from which intermediate frequency voltage is obtainable, iscoupled through a condenser 43 to one end of a two-branch tuned circuit,the branches being designated A and B. Branch A consists of a capacity44a in series with an inductance 45a, and. branch B which is in parallelwith branch A likewise comprises series elements Mb and 45b. The twobranch circuit is tuned as a whole to the operating intermediatefrequency, but one branch is arranged to be an acceptor circuit for afrequency above the operating intermediate frequency and; the other tobe an acceptor circuit for a frequency a corresponding amount below theintermediate-frequency. For example, if the intermediate frequency is100 kilocycles one branch may be tuned to 110 kilocycles and the otherto. 90 kilocycles. The junction of the inductance 45a and the capacityNa in branch A is connected through a rectifier 46afor example a copperoxide rectifier in series with a condenser shunted resistancecombination 47a i80L---to that end of the two branch circuit remote fromthe coupling condenser 43, and in a similar manner the junction point ofinductance 452) with capacity 44b is connected through a secondrectifier 46b and a second capacity shunted resistance combination llb58b to the same end of the two branch circuit. Adjustable tapping points49a and 4% upon the resistance 41a 41b respectively, are connectedtogether through a further resistance 50,'and the mid-point of this.further resistance is connected through a still further resistance 51 tothe first grid 3% of a valve 3| which is again a pentode and which actsas a variable reactance device. The rectifiers are connected in oppositesense so that one of the two adjustable tappingw points 4911 or will bepositive and the other negative as a resultv of uni-directional currentsobtained from rectification. The end of the two branch circuit remotefrom the intermediate frequency amplifier is connected to an adjustabletapping point 3'! upon the potentiometer resistance 38, which as in thepreviously described embodiment is shunted between the negative andpositive terminals 83, ll, respectively, of a source of potential (notshown) the positive terminal of this source being connected, to thesecond grid 34 of the pentode andthe'tapping point 31 being connected tothe cathode 35 of the pentode. The cathode 35 is also connected as in.the usual way to the third or suppressor grid 36 of the pentode. Theanode 33 of the pentode is, as before, connected through a resistance 40in series with a choke 39 to the positive terminal I 5 of a source ofanode potential and a variable condenser 32 is connected between thecontrol. grid 38v and the anode 33. The control grid-cathode space ofthe valve 3| is connected by means of. terminals I3; 60, across thenormally provided coil (not shown) in the adjustable frequencydetermining circuit of the local" oscillator of the receiver.

In practice the local oscillator is manually adjusted as in the usualway, so as to beat with received signals to produce as nearly aspossible the desired operating. intermediate frequency, but in a circuitas just described, owing to the interconnectionof the pentode with thelocal oscillator of thereceiver, if this manual adjustment is notaccurately made, or if the received signal drifts somewhat in frequency,the local oscillator tuning will be, automatically varied in such,manner as to produce the correct operating intermediate frequency. Inthis connection it will be appreciated that so long as the intermediatefrequency is accurately obtained at the point from which it is taken offto the two branch circuit, the potentials set up by the two rectifierswill be equal and the bias upon the first grid of the pentode will notbe changed. faulty manual tuning or by reason of drift in the receivedsignal frequency, the proper beat frequency is not obtained, the biasupon the pentode will be changed and this change will produce avariation in the effective reactance in the frequency determiningcircuit of the local oscillator.

in such direction as to rectify the fault. In this way the action of thesignal itself is utilized to correct for frequency drift or for tuninginaccuracies. Accordingly, the invention enables a rela- 1 tively narrowband pass intermediate frequency amplifier to be employed since accuratelocal oscillator tuning is automatically obtained, while furthermore, inreceivers having automatic gain control the tuning indicator meansusually deemed necessary to assist accurate manual tun ing, can beomitted.

The last described embodiment is satisfactory so long as a normal fairlylow intermediate or beat frequency is employed e. g. an intermediatefrequency of the order of 100 kilocycles, but where very high beatfrequencies are employed copper oxide rectifiers are not satisfactoryand the modification illustrated in Figure 3 is preferably employed. Inthis figure, in which parts corresponding to like parts in Figure 2 areindicated by like characters, a two branch circuit is used as beforeand, also as before, one branch is tuned slightly above and the otherslightly below the operating beat frequency which is now presumed to beof a high value of, for example, 2,000,000 cycles per second. For2,000,000 cycles per second one branch might be tuned to 2,020,000cycles and the other to 1,930,000 cycles. One end of this twobranchcircuit is, as before, connected through a coupling condenser to thebeat frequency ampliher; and the other end is connected to the cathodeof a double diode triode iii. A point between the inductance andcapacity in one branch of the twobranch circuit is connected through acapacity shunted resistance combination 41a, 48a to one diode anode iterof the double-diode-triode tube 3i-this anode ta' is also connected tothe grid 6? of the double-diode-triode-while a corresponding point onthe other branch of the twobranch circuit is connected through acapacity shunted resistance combination llb 48b to the second diodeanode 4611, this second diode anode being connected through a resistance50' to an If, however, either by reason of,

adjustable tapping point 68 upon a further capacity shunted resistancecombination 69, 10, which is in series between the cathode H of thedoublediode-triode and the negative terminal l3 of an anode potentialsource. The anode 12 of the double-diode-triode is connected through aresistance 13 to the positive terminal ll of the source of anodepotential, and the mid-point of the resistance 50' is connected througha further resistance to the control grid 30 of the valve 3| which actsas a variable reactance valve. The cathode 50 of this valve is connectedas before to an adjustable tapping point 3'! upon a resistance 38shunted between the, terminals IS, H, the remaining connections beingsimilar to those of Figure 2 The invention is not limited to itsapplication to heterodyne receivers and the principles embodied in theself-tuning super-heterodyne receivers described are applicable in likemanner to securing self-tuning action against slight inaccuracies oftuning or slight signal frequency drifts in other types of receiversforexample, receivers embodying ordinary radio frequency tuned amplifiers.

Having now particularly described and ascertained the nature of my saidinvention and in what manner the same is to be performed, I declare thatwhat I claim is:-

In a superheterodyne receiver of the type pro- Vided with anintermediate frequency energy circuit and a tunable local oscillatorcircuit whose frequency is to be automatically controlled, theimprovement which consists of a network comprising a pair of resonantcircuit-s, each resonant circuit including a rectifier, means forimpressing the intermediate frequency energy upon said resonantcircuits, one of the resonant circuits being tuned to a frequencydiffering from the intermediate frequency by a predetermined frequencyvalue, the other resonant circuit being tuned to a frequency differingfrom the intermediate frequency by the same frequency value but in,an

' opposite direction, an electron discharge tube,

means, responsive to the current flow in one of said rectifier circuits,for varying the space current flow of said tube, an impedance in thespace current circuit of said tube, and tube means connected to saidimpedance and the second of said rectifier circuits, and Whose gain isresponsive to direct current voltage developed across said im pedance bythe space current flow therethrough and the current flow in said secondrectifier circuit, for providing a reactance of variable magnitude.

NOEL MEYER RUST.

